Solar cells are used to convert sunlight into electricity using a photovoltaic effect. A general object is to achieve high conversion efficiency balanced by a need for low production costs.
In a solar cell, emitter regions and base regions are comprised in a semiconductor substrate and a junction between these oppositely doped regions serves for separating light-generated charge couples. Therein, an emitter region comprises one of n-type and p-type doping whereas a base region comprises the opposite doping type.
In conventional solar cells, the emitter region is typically arranged at a top surface directed towards the impinging light and the base region is arranged at the opposite rear surface of the semiconductor substrate. Accordingly, emitter contacts contacting the emitter region are arranged on the front side and base contacts contacting the base region are arranged on the rear side of the semiconductor substrate.
Novel cell designs have been developed wherein both contact types are arranged on a rear surface of the semiconductor substrate. Such solar cells are typically referred to as rear contact solar cells.
One type of rear contact solar cells which will be mainly discussed herein is the metal wrap-through (MWT) solar cell design in which an emitter region is formed at the front side of the semiconductor substrate and small metal fingers are arranged on this front side to form emitter contacts. However, in contrast to conventional solar cells, these small metal fingers do not lead to larger fingers, arranged perpendicular to the small fingers and typically referred to as busbars. Instead, these small fingers lead to through-holes generated all over the area of the semiconductor substrate. These through-holes are filled with metal thereby connecting the small front side fingers with an emitter contact region arranged on the rear surface of the semiconductor substrate. Accordingly, the MWT-cell may have both contact types on the rear surface such that no light-shading busbars are needed on the front side.
On each of the emitter contacts and base contacts on the rear side surface, soldering pad arrangements may be applied. These soldering pad arrangements may comprise one or more soldering pads made with a solderable material such as e.g. silver. On top of such soldering pads, interconnecting structures may be soldered for interconnecting neighboring solar cells thereby forming interconnected strings of solar cells which may then be used for finally forming a solar module.
Several concepts and interconnection schemes have been developed for electrically interconnecting a plurality of rear contact solar cells. General requirements to be fulfilled by such approaches and schemes are that no short-circuits shall occur between emitter regions and base regions via the applied interconnection structures. Furthermore, the interconnecting structures shall be easy to be applied to the soldering pad arrangements without significant risk of damaging the solar cell by the interconnection procedure. Furthermore, processing steps and materials used for interconnecting solar cells to a string should be as simple and cheap as possible.